Four-wire speakerphone circuits using voice switch control are arranged to automatically switch to either a transmit mode or a receive mode based on the level of speech energy present in the respective transmission paths. In prior art circuits, the voice switching decision is made by comparing a Transmit Talk Down (TTD) signal against a Receive Talk Down (RTD) signal. Improvements in speakerphone circuitry led to the inclusion of a Receive Signal Guard (RSG) signal and a Transmit Noise Guard (TNG) signal in the comparison process. These signals account for electro acoustic noise coupled from the loudspeaker to the microphone and background noise received by the microphone and prevent the speakerphone from self-switching. However, the inclusion of these two additional signals in the comparison process degraded the sensitivity of the voice switch.
An unfavorable aspect of degraded voice switch sensitivity is that it allows a loud talker to maintain control of the voice switch and the conversation. As such, the opposite party's speech energy must greatly exceed the talker's speech energy to inject a response. This problem has been further aggravated by the inclusion of a Transmit Switch Guard (TSG) signal in the voice switch comparator to account for signals coupled from the transmit path to the receive path via the speakerphone's four-wire to two-wire hybrid circuit.
Conventional arrangements directed at improving the sensitivity of the voice switch circuit have not been successful, since these circuits still allow loud talker domination of the voice switch and are still unable to distinguish small differences between the level of talk down signals in the presence of switch guard signals. Consequently, as a result of loud talker domination, a short burst of incoming speech signals appearing on the receive channel during a pause in transmitted speech, as is generally the case during natural conversation, does not cause the voice switch to transfer to the receive mode.
The lack of success on the part of prior art circuits to improve the sensitivity of the voice switch circuits is due in part to comparing talk down signals against switch guard signals to effect a switching decision, rather than basing the switching decision on the presence of legitimate signals. Thus, prior art circuits retain high switching thresholds to prevent a speakerphone circuit from self-switching in response to noise and switch guard signals.
As mentioned, the lack of switching sensitivity on the part of prior art circuits has been compounded by the inclusion of a Transmit Switch Guard signal in the comparison process. This aspect of voice switching was first suggested by A. Busala in his article entitled "Fundamental Considerations in the Design of a Voice Switched Speakerphone", dated March, 1960 and published in Vol. 39 at page 266 of the Bell System Technical Journal. An example of the art taught by Busala is disclosed in U.S. Pat. No. 4,002,954, issued to D. J. Penrose on Jan. 11, 1977. Penrose discloses a speakerphone arrangement in which unidirectional signals, derived from the speakerphone transmit and receive channels, are compared to produce a control signal. The control signal, which is based on the strength of the signal being compared rather than on the legitimacy of those signals, is used by Penrose to control the speakerphone transmission mode. In this respect, the Penrose speakerphone circuit is not representative of an advancement over the prior art and is essentially a continuation of that art.
Accordingly, a general object of this invention is to significantly improve the sensitivity of speakerphone circuits in order to promote natural conversation and eliminate loud talker control.